Photos are greyed out - "files not accessible"

All my photos are linked (no errors) to my external hard drive. Some are jpgs, pngs and heic.
Problem:

- Some photos are greyed out. They have nothing in common and have random file types.

- Some photos seem to be dragged down and minimized (see screenshot), so weird.

When I replace the greyed out and "dragged down" photos, with the already linked photo with no errors and correct link, they look good.

Will I have to do this with all the (hundreds) of photos?
Why is there an error when the links are correct?
Could the problem be my external harddrive?